Dye-sensitized solar cell

ABSTRACT

A dye-sensitized solar cell with high conversion efficiency is provided. The dye-sensitized solar cell according to the present invention has, between an electrode ( 2 ) formed on a surface of a transparent substrate ( 1 ) and a counter electrode ( 6 ), a light-absorbing layer ( 3 ) containing light-absorbing particles carrying dye and an electrolyte layer ( 5 ), characterized in that the light-absorbing layer ( 3 ) containing light-scattering particles ( 4 ) different in size from the light-absorbing particles. In such a dye-sensitized solar cell according to the present invention, the energy of light, which passes through a light-absorbing layer in a conventional cell structure, can be strongly absorbed by the dye in the light-absorbing layer of the present invention. This will increase the conversion efficiency and output current of the dye-sensitized solar cell.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 10/480,604,filed on Jul. 22, 2004, entitled SENSITIZING DYE SOLAR CELL which, inturn, claims priority under 35 U.S.C. 371 and 37 CFR 1.495 toPCT/JP03/04518, filed Apr. 9, 2003, which, in turn, claims priority toJapanese application No. JP2002-109408 filed Apr. 11, 2002.

TECHNICAL FIELD

The present invention relates to a dye-sensitized solar cell andparticularly to a dye-sensitized solar cell with an improved conversionefficiency expressed by the ratio of the cell output to the quantity ofincident light.

BACKGROUND ART

In Japan, photovoltaic power generation systems are now beingcommercially introduced and coming into widespread use. Although somep-n junction silicon solar cells are available on the market, theirelectricity prices are much higher than those of distribution lines.Accordingly, inexpensive solar cells are required for the proliferationof photovoltaic power generation systems. FIG. 3 is a schematic diagramshowing a cross-sectional structure of a conventional dye-sensitizedsolar cell. Reference numeral 101 represents a glass substrate, andreference numeral 102 represents an electrode that is provided on thelower surface of the glass substrate 101 and that is composed of atransparent material, for example, tin oxide (SnO₂) doped with fluorine(F). Reference numeral 103 represents a light-absorbing layer of finesemiconductor particles 103 a deposited on the electrode 102 andcomposed of titanium oxide with a particle size of about 50 nm or less,dye 103 b being adsorbed on the surfaces of the fine semiconductorparticles. The light-absorbing layer 103 is formed in a film about 10 μmor less in thickness. An electrolyte 104 includes or infiltrates thelight-absorbing layer 103. Reference numeral 105 represents acounter-electrode.

Such a dye-sensitized solar cell, as compared with a semiconductor p-njunction solar cell, has an advantage that it can convert light atlonger wavelengths into electricity. In addition, the dye-sensitizedsolar cell has another advantage in that it can be manufactured usingless energy than the p-n junction solar cell, which requiresconsiderable energy for its manufacture.

While the dye-sensitized solar cell has such advantages, however, itslow conversion efficiency stands in the way of commercialization. Thislow conversion efficiency is caused by high transmission of light oflong wavelengths in the light-absorbing layer 103 because of the verylow light absorptivity of the dye 103 b. To improve the conversionefficiency, the light-absorbing layer 103 composed of the finesemiconductor particles 103 a carrying dye 103 b may be increased inthickness. However, a thicker light-absorbing layer 103 will cause aproblem of further reduced conversion efficiency, because thelight-absorbing layer 103 structurally has high series resistance.

In contrast, reducing the thickness of the light-absorbing layer 103 todecrease the series resistance will increase the transmission oflong-wavelength light, lowering the light absorption efficiency. In bothcases, therefore, it is difficult to increase the conversion efficiency.Consequently, a dye-sensitized solar cell that satisfies thesecontradictive requirements for conversion efficiency and lightabsorption efficiency as described above has not yet been developed.

In light of the existing problems described above, an object of thepresent invention is to provide a dye-sensitized solar cell with highconversion efficiency.

DISCLOSURE OF INVENTION

A dye-sensitized solar cell according to the present inventioncomprises, between an electrode formed on a transparent substrate andthe counter electrode, a light-absorbing layer including light-absorbingparticles carrying sensitizing dye, and an electrolyte layer,characterized in that the light-absorbing layer containslight-scattering particles different in size from the light-absorbingparticles.

In such a dye-sensitized solar cell according to the present invention,the addition of light-scattering particles that are different in sizefrom the light-absorbing particles to the light-absorbing layer causesincident light on a light-collecting layer to be adequately scattered,thus increasing the optical path length through the light-absorbinglayer. This enhances the absorption of the scattered light by the dyeadsorbed on the light-absorbing particles and leads to greatly increasedlight absorption. As a result, the energy of light, which passes throughthe light-absorbing layer in the conventional cell structure, can bestrongly absorbed by the dye in the light-absorbing layer. This willincrease the conversion efficiency and output current of thedye-sensitized solar cell.

Moreover, the dye-sensitized solar cell that has such a structureaccording to this invention does not require a thicker light-absorbinglayer for higher conversion efficiency; therefore, the series resistanceof the light-absorbing layer is not increased and thus reduction in theconversion efficiency due to the increased thickness of thelight-absorbing layer does not occur. Accordingly, the conversionefficiency can be improved while maintaining the small thickness of thelight-absorbing layer.

Furthermore, these effects are particularly noticeable forlong-wavelength light. Low conversion efficiency due to hightransmission of long-wavelength light can be compensated for whilemaintaining the small thickness of the light-absorbing layer, and thusthe conversion efficiency can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the structure of adye-sensitized solar cell according to the present invention.

FIG. 2 is a characteristic graph showing the relationship between thediameter and the volume fraction of light-scattering particles at alight-scattering rate of 50% where a light-absorbing layer 10 μm inthickness is irradiated with light at a wavelength of 600 nm.

FIG. 3 is a cross-sectional view showing the structure of a conventionaldye-sensitized solar cell.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in detail with reference tothe drawings. This invention is not limited to the description below andmay be modified as appropriate without departing from the gist of theinvention.

A dye-sensitized solar cell according to the present inventioncomprises, between an electrode formed on a transparent substrate andthe counter electrode, a light-absorbing layer including light-absorbingparticles carrying sensitizing dye, and an electrolyte layer,characterized in that the light-absorbing layer containslight-scattering particles different in size from the light-absorbingparticles.

The size of particles in this invention may be determinedmicroscopically or as a mean particle diameter.

FIG. 1 is a schematic cross-sectional view showing the cross-sectionalstructure of a dye-sensitized solar cell according to the presentinvention. The dye-sensitized solar cell includes a transparentsubstrate 1, an electrode 2, a light-absorbing layer 3, light-scatteringparticles 4, an electrolyte layer 5, and a counter electrode 6.

The transparent substrate 1 is not particularly limited and may be aglass substrate.

The electrode 2 is composed of a transparent material and is formed onthe lower surface of the transparent substrate 1. Any material that iselectrically conductive and transparent may be used in the electrode 2.Tin oxide may be suitable due to its high electrical conductivity,transparency, and heat-resistance, and indium tin oxide (ITO) ispreferred in view of cost.

The light-absorbing layer 3 contains light-absorbing particles depositedon the electrode 2. The light-absorbing particles carry dye 3 b thatabsorbs light incident on the light-absorbing layer 3 from thetransparent substrate 1 through the electrode 2. The light-absorbingparticles are fine semiconductor particles 3 a that can adsorb the dye 3b on their surfaces. The fine semiconductor particles 3 a may becomposed of elemental semiconductors, such as silicon semiconductors,compound semiconductors, or compounds with a perovskite structure.Preferably, these semiconductors are n-type semiconductors, in whichconduction band electrons behave as carriers under photoexcitation togenerate an anodic current. Exemplary semiconductors are, but notlimited to, titanium dioxide (titania, TiO₂), tin dioxide (SnO₂), zincoxide (ZnO), tungsten trioxide (WO₃), niobium oxide (Nb₂O₅), andtitanium strontium oxide (TiSrO₃). In particular, the semiconductor ispreferably anatase TiO₂. These semiconductors may be used alone or incombination. In addition, the light-absorbing particles preferably havea large surface area to allow an increased quantity of light to beabsorbed; hence, the diameter of the fine semiconductor particles ispreferably about 20 nm or less.

Preferably, an example of the dye 3 b that is adsorbed on the finesemiconductor particles is, but not limited to, ruthenium dye. As longas it functions as a charge separator and sensitizer, the dye 3 b may beof any type. Thus, in addition to the ruthenium dye, xanthene dyes, suchas rhodamine B, rose bengal, eosin, and erythrosine; cyanine dyes, suchas quinocyanine and kryptocyanine; basic dyes, such as phenosafranine,fog blue, thiosine, and methylene blue; porphyrins, such as chlorophyll,zinc porphyrin, and magnesium porphyrin; azo dyes; phthalocyaninecompounds; complex compounds, such as ruthenium trisbipyridyl;anthraquinone dyes; and polycyclic quinone dyes may be used alone or incombination.

The light-absorbing layer 3 preferably has a thickness of 15 μm or less.The light-absorbing layer 3 structurally has high series resistance,which causes low conversion efficiency. The light-absorbing layer 3 witha thickness of 15 μm or less achieves low series resistance of thelight-absorbing layer 3, while the absorptivity of the light-absorbinglayer 3 is maintained. This prevents a reduction in the conversionefficiency.

The light-scattering particles 4 that are contained in thelight-absorbing layer 3 scatter light incident on the light-absorbinglayer 3 from the transparent substrate 1 through the electrode 2. Thelight-scattering particles 4 are different in size from thelight-absorbing particles. More precisely, the light-scatteringparticles 4 are larger than the light-absorbing particles. Addition ofsuch light-scattering particles 4 into the light-absorbing layer 3allows the light incident on the light-absorbing layer 3 to be scatteredadequately. The scattered light has a much larger optical path length inthe light-absorbing layer 3 than that of light directly passing throughthe light-absorbing layer 3. This enhances the absorption of light bythe dye 3b on the light-absorbing particles and allows a much largerquantity of light to be absorbed. Thus, a much lower quantity of lightreaches the electrolyte layer through the light-absorbing layer 3. Thiscompensates for the low absorptivity of the dye 3b and thus increasesthe conversion efficiency.

These effects enhance the conversion efficiency without increasing thethickness of the light-absorbing layer 3. This prevents the seriesresistance of the light-absorbing layer 3 from increasing and does notcause low conversion efficiency due to increased series resistance.Accordingly, it is possible to enhance the conversion efficiency with athin light-absorbing layer 3.

The above effects are particularly noticeable in long-wavelength light,compensate for the low conversion efficiency resulting from hightransmission of the long-wavelength light, and can enhance theconversion efficiency.

Such light-scattering particles 4 have a particle diameter from about 20nm to 100 nm, and are made of high-refractive-index material, such asrutile titanium dioxide. By designing the particle size of thelight-scattering particles 4 to be slightly larger than one-twentieth ofthe target wavelength, it is possible to ensure scattering of lightregardless of the small size of the light-scattering particles.

The light-scattering particles 4 and the light-absorbing particles maybe made of the same material. By carrying the dye 3 b on thelight-scattering particles 4 made of the same material as used in thelight-absorbing particles, the light-scattering particles 4 not onlyscatter the light incident on the light-absorbing layer 3, but alsoabsorb light.

In addition, the light-scattering particles 4 may be made of a materialdifferent from that used in the light-absorbing particles. Even in thiscase, by carrying the dye 3 b on the material that is different from thematerial used in the light-absorbing particles but that can be used forthe light-absorbing particles, the light-scattering particles 4 not onlyscatter the light incident on the light-absorbing layer 3, but alsoabsorb light.

As described above, providing the light-absorbing layer 3 containing thelight-scattering particles 4, rather than providing another layerconsisting solely of the light-scattering particles 4, allows effectiveuse of space by filling the vacant spaces among the largelight-scattering particles 4 with the small light-absorbing particles.As described above, the light-scattering particles 4 themselves also canserve as light-absorbing particles. Therefore, these improve thelight-absorption coefficient and can reduce the thickness of thedye-sensitized solar cell.

The electrolyte layer 5 contains an electrolyte and may include thelight-absorbing layer 3 or may be provided such that the electrolyteinfiltrates the light-absorbing layer 3. An example of the electrolyteis, but not limited to, a propylene carbonate solution of iodine or anyknown electrolyte having a hole conducting function.

The counter electrode 6 may be made of any conductive substance. It mayalso be made of an insulating substance having a conductive layer thatfaces a semiconductor electrode. Preferably, the counter electrode maybe made of an electrochemically stable material, in particular,platinum, gold, carbon, and so on. Moreover, to enhance redox catalyticeffects, the surface of the counter electrode that faces thesemiconductor electrode preferably has a microstructure and therefore anincreased surface area. For example, platinum is preferably platinumblack and carbon is preferably porous carbon. Platinum black may beformed by anodic oxidation coating of platinum, treatment ofchloroplatinic acid, or the like. Porous carbon may be formed bysintering of fine carbon particles, burning of organic polymers, or thelike.

The structure of such a dye-sensitized solar cell according to thepresent invention is characterized in that, as described above, theaddition of the light-scattering particles 4 into the light-absorbinglayer 3 causes light that is incident from the transparent substrate 1and passes through the light-absorbing layer 3 to be more stronglyabsorbed by the dye 3 b. Addition of the light-scattering particles 4 tothe light-absorbing layer 3 allows light incident from the transparentsubstrate 1 to be scattered sufficiently and thus increases the opticalpath length through the light-absorbing layer 3. As a result, thescattered light is easily absorbed by the dye 3 b on the light-absorbingparticles, and the quantity of light absorbed by the dye 3 b on thelight-absorbing particles is much increased. This allows much of theenergy of light, which passes through the light-absorbing layer 3 in theconventional structure, to be absorbed by the dye 3 b in thelight-absorbing layer 3. Accordingly, the conversion efficiency andtherefore the output current of the dye-sensitized solar cell can beincreased.

These effects enhance the conversion efficiency without increasing thethickness of the light-absorbing layer 3. This prevents the seriesresistance of the light-absorbing layer 3 from increasing and does notcause low conversion efficiency due to increased series resistance.Accordingly, it is possible to enhance the conversion efficiency with athin light-absorbing layer 3.

Furthermore, these effects are particularly noticeable inlong-wavelength light. The low conversion efficiency due to the hightransmission of long-wavelength light can be compensated for whilemaintaining the reduced thickness of the light-absorbing layer 3 andthus the conversion efficiency can be increased. Therefore, it ispossible to produce a dye-sensitized solar cell that has a thinlight-absorbing layer 3 while satisfying the requirements for conversionefficiency and light absorption efficiency.

The following describes the light-scattering rate of the light-absorbinglayer 3 in the dye-sensitized solar cell according to the presentinvention as described above, the light-scattering particles 4, and thevolume fraction of the light-scattering particles 4 in thelight-absorbing layer 3. A dye-sensitized solar cell according to thepresent invention was prepared as shown in FIG. 1. The details of thedye-sensitized solar cell are as follows. The transparent substrate 1used was a glass substrate. Indium tin oxide (ITO) was deposited on amain surface of the transparent substrate 1 to form the electrode 2.Light-absorbing particles were stacked on the electrode 2 to form thelight-absorbing layer 3 with a thickness of 10 μm. TiO₂ particles with aparticle size of 10 nm were used as the fine semiconductor particles 3a. The light-absorbing particles were formed by adsorbing ruthenium dyeon the TiO₂ particles. The light-absorbing layer 3 had light-scatteringparticles 4 added thereto. A propylene carbonate solution of iodine wasused in the electrolyte layer 5. The counter electrode was made ofplatinum.

The relation between the diameter and the volume fraction of thelight-scattering particles was studied where such a dye-sensitized solarcell was irradiated with light at a wavelength of 600 nm and thelight-scattering rate by Rayleigh scattering was 50%. FIG. 2 shows theresult. The light-absorbing layer 3 in the presence of thelight-scattering particles absorbs half the transmitted light in theabsence of the light-scattering particles, and accordingly thelight-absorbing rate is increased. When the light-scattering rate is 50%or less, the cell is only slightly improved in performance and of nopractical use. When the light-scattering rate exceeds 50%, improvementin the conversion efficiency is significant. If a light-scatteringefficiency of 50% is achieved, it is possible to obtain high conversionefficiency and produce a dye-sensitized solar cell for practical use.

The light-scattering efficiency depends on the size and volume fractionof the light-scattering particles. The volume fraction of close-packedspheres is about 66%. When the light-scattering efficiency is 50%, thesize of the light-scattering particles for achieving maximum packing isabout 20 nm. Therefore, light-scattering particles not less than 20 nmwill ensure a light-scattering efficiency of 50%. On the other hand,when the size of the light-scattering particles is 100 nm or larger, thefraction of the light-scattering particles in the light-absorbing layer3 becomes too large and the fraction of light-absorbing particles in thelight-absorbing layer 3 becomes too small. As a result, the surface areain the light-absorbing layer 3, or area for absorbing light becomes toosmall. This lowers the light conversion efficiency of the dye-sensitizedsolar cell and thus makes the cell of no practical use. Accordingly, asdescribed above, the size of the light-scattering particles 4 ispreferably about 20 nm to 100 nm.

Japanese Unexamined Patent Application Publication No. 10-255863discloses a dye-sensitized solar cell in which a light-absorbing layercontaining small light-absorbing particles is disposed on a layercontaining large light-reflecting particles. An experiment demonstratedthat the maximum light absorptivity was only 0.8 in this case, while itwas 1.0 for the dye-sensitized solar cell according to the presentinvention.

A dye-sensitized solar cell according to the present inventioncomprises, between an electrode formed on a transparent substrate andthe counter electrode, a light-absorbing layer including light-absorbingparticles carrying sensitizing dye, and an electrolyte layer, whereinthe light-absorbing layer contains light-scattering particles differentin size from the light-absorbing particles.

With a thus-constituted dye-sensitized solar cell according to thepresent invention, the absorption of light by the light-absorbing layer3 can be increased without increasing the thickness of the layer orincreasing the series resistance of the layer. Therefore, it is possibleto produce a dye-sensitized solar cell that has a thin light-absorbinglayer and satisfies the requirements for conversion efficiency and lightabsorption efficiency. The dye-sensitized solar cell can absorb much ofthe energy of light, which passes through the light-absorbing layer inthe conventional cell. Thus, the output current of the dye-sensitizedsolar cell can be increased.

Accordingly, the present invention provides a dye-sensitized solar cellwith high conversion efficiency.

1. A photocell comprising, between an electrode formed on a surface of asubstrate and a counter electrode: light-absorbing particles carryingdye, light-scattering particles different in size from thelight-absorbing particles, and an electrolyte; wherein a particle sizeof the light-scattering particles is larger than a particle size of thelight-absorbing particles; and wherein the particle size of thelight-absorbing particles is 20 nm or less.